For the purpose of this invention, poorly water soluble and highly lipophilic camptothecin derivatives (referred to as "HLCD" for the purposes of this invention) are defined interchangeably as any unsubstituted or any A-ring and/or B-ring substituted camptothecin which has a water solubility of less than 5 micrograms per milliliter (&lt;5 .mu.g/mL) of water. Also for the purposes of the instant invention, the terms "highly lipophilic" and "poorly water soluble" are used interchangeably to describe the fundamental bioavailability and chemical behavior of the camptothecin derivatives.
Utilizing HPLC and NMR techniques, researchers have demonstrated that camptothecin and many of its derivatives undergo an alkaline, pH-dependent hydrolysis of the E-ring lactone. The slow reaction kinetics allow one to assess whether both the lactone and non-lactone forms of the drug stabilize the topoisomerase I-cleaved DNA complex. Studies indicate that only the closed lactone form of the drug helps stabilize the cleavable complex. This observation provides reasoning for the high degree of drug activity observed in solid tumor models. Tumor cells, particularly hypoxic cells prevalent in solid neoplasms, have relatively lower intracellular pH levels than normal cells. At pH levels below 7.0, the lactone E-ring form of camptothecin predominates.
Formulations of camptothecin and its derivatives in the lactone form are difficult to prepare, due to the factors cited above. The poor solubility of these compounds in aqueous solution prohibits administration of effective doses. The opening of the lactone ring in alkaline formulations precludes their utility as well, due to a substantial reduction in the anti-tumor potency of the compounds.
The prior art teaches the use of various organic solvents useful for camptothecin formulations. This prior art is identified in the Information Disclosure Statement accompanying this application. Such solvents include lipid-based oils, such as cottonseed oil, peanut oil, IL-20 and others, and organic solvents such as N,N-dimethylacetamide (DMA), dimethylisosorbide (DMI), and others. Solubility of the compounds in lipid-based solvents is generally less than 1 mg/mL, while the solubility increases to as high as about 6.7 mg/mL in certain organic solvents.